---
title: Version detection
toc_max_heading_level: 6
---

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<HeaderLabel text="2 min" />

The most powerful feature in proto is its contextual version detection, that is triggered with
[`proto run`](./commands/run), [`proto bin`](./commands/bin), or when a shim is executed. So what
does this mean exactly? Before a tool in proto's toolchain can be executed, we need to determine the
version of the tool to execute with. If a detected version exists locally, we proceed using that
binary, otherwise we fail with a missing installation error.

When detecting a version, the following steps are checked, in the order as listed:

#### 1. Version is explicitly passed as a command line argument

```
$ proto run node 18.0.0
```

#### 2. Version is provided with the `PROTO_*_VERSION` environment variable

```
$ PROTO_NODE_VERSION=18.0.0 proto run node
```

#### 3. Version is located by traversing the file system

This step will attempt to find a configuration or manifest file in the current working directory,
and traverse upwards through parent directories (stops at the user's home directory) until a file is
found.

##### 3.1. Version is defined locally in `.prototools`

A `.prototools` file was found and a version entry exists for the current tool. This is also known
as a "local version" and can be created with [`proto pin`](./commands/pin).

```toml title=".prototools"
node = "18.0.0"
```

##### 3.2. Version is defined in the tool's ecosystem

Depending on the tool, a version is extracted from a found file unique to that tool's ecosystem.
This includes version manager configs (`.nvmrc`, etc), manifest files (`package.json`, etc), and
more.

```text title=".nvmrc"
18.0.0
```

#### 4. Version is defined globally

As the last check, we look for a "global version" that was pinned with
[`proto pin --global`](./commands/pin) or [`proto install --pin`](./commands/install). This version
is stored at `~/.proto/.prototools`.

#### 5. Version _could not_ be detected

If all the previous steps have failed, then we could not detect an applicable version, and the
process will fail.
